You are doing amazingly well aleC, such a strong dignified lady, and a great mum.
I found work a great help, supportive colleagues and teaching means we have to live in the moment, so I was forced to focus on my job, which really helped.
Your comment about your ex having an idealised idea of marriage really resonated too; my ex came from a broken home and looking back I think he wasn't realistic about the reality of day to day married life. I think your ex, like mine, will realise that once the novelty wears off, the grass isn't greener.
My ex doesn't get that happiness comes from within, and it isn't up to other people to make him happy. Ultimately he will always be unhappy, moving to others, searching for the impossible. Blaming them when they can't live up to his impossible expectations.
Your mum, like my parents is worried about you and the kids. Mine couldn't understand how mine walked away, and they also had to grieve by losing, without warning, the man they had treated like a son for more than twenty years. What helped them was seeing that I was coping. I think my little eighty year old mum would wipe the floor with him if their paths crossed again!
Have a good day, hope DS settles well in his new school.